How bad is the pollution in Beijing? So much so that the city has been put on red alert.

In 2013, the Chinese capital instituted a red-alert system to keep people indoors on days when air pollution climbs to hazardous levels. Specifically, an issued red alert dramatically limits car use, advises schools to close, shuts down outdoor construction sites and closes certain industrial plants.

But the government did not issue an actual red alert for the next two years — the first time they used the system was earlier this month. Now they have issued their first two only weeks apart from each other.

After announcing its first red alert on Dec. 7, Beijing's local government is once again advising locals to stay inside from Saturday through Tuesday, and will be restricting the amount of traffic allowed on roads, according to a tweet posted by CCTV News, one of the major state-controlled broadcasters in China.

The announcements appear to be signs of the government's growing desire to show an increasingly vocal public that it is taking steps to address the environmental problems that have attended China's economic growth.

Coal is one of the major fuels that has literally powered that growth — China is the world's largest user of coal, and some evidence suggests the country may be burning even more of it than previously thought.

Fossil fuels, industrialization and a dramatic rise in driving have all left the air nearly unbreathable in many areas, and the red alert system was a way to reduce pollution and protect health on days when the smog is expected to be especially bad.

But why is the government suddenly issuing red alerts, after letting the program lie fallow for so long?

"I was quite surprised they issued a second red alert so soon after the first one," said Jennifer Turner, who heads the China Environment Forum at the Woodrow Wilson Center. But she thinks there could be a few reasons for the spike.

First, the government has been working on improving its air quality-monitoring technology since its air pollution became a news story a few years ago. They might feel more confident in their measurements now than they have in the past, she said.

Secondly, Beijing's air quality is awful, and the red alerts may be a way for the government to tell the public that further legal or policy changes lie ahead. "They could have some tough decisions ahead of them, and it may be a way to catalyze more action on this," she said.

Thirdly, they might be responding to louder and louder signals of discontent from the Chinese public.

Indeed, the Chinese people are increasingly upset about air and water quality and other environmental issues, and the government is likely feeling some pressure to react, said Steve Herz, an attorney with the Sierra Club, an environmental advocacy group.

"China has a massive air pollution problem, and it has turned into a political problem," Hertz told CNBC. "The government of China values more than anything stability and legitimacy, and it has become such a problem that they need to be seen to be acting to address it."

The pollution in the city has become so hazardous, it is common to see people wear face masks to filter out the pollution, and air pollution is cited as one of the reasons many of China's newly wealthy citizens are leaving the country for the United States, Australia or other countries.

Of specific concern is a type of pollution known as PM2.5 — particulate matter up to 2.5 micrometers in size. These particles are small enough that they can enter the body through the lungs and make their way into the bloodstream. PM2.5 is considered a carcinogen and a contributor to cardiovascular disease by the U.S. Environmental Protection Agency.

One study published earlier this year estimated that 1.6 million people died premature deaths in a single year from air pollution-related conditions.

The pollution has also become an economic problem. Many businesses will likely have to close during the red alert, certain types of large vehicles are not permitted on the roads, and about half of the city's cars will have to stay in the garage.

"You often hear people talk about coal as a cheap energy source. First of all, that is increasingly not true, when you look at the plummeting costs of renewables, and the costs of building new coal plants. But even [if] you take that as true, you have to think about the hidden costs. Think of the hidden health-care costs, not just the 1.6 million who are dying, but the millions and millions who are sickened with asthma attacks and cardiopulmonary disease. I mean the health-care costs in the Chinese system must be staggering."

Herz compared the red alerts with other steps he has seen at varying levels of government in China. Earllier this year, China committed to peak its carbon emissions by 2030, and to cut carbon emissions per unit of GDP by at least 60 percent of what they were in 2005.

They have already begun to shut down some of the least efficient fossil fuel-burning power plants, and have committed about $6 trillion to clean energy.

The country has also implemented a cap-and-trade system for fossil fuel emissions, said Herz, and plans to make changes to power grids to prioritize the delivery of electricity from renewable sources over power from fossil fuels.

"It may the first time they are issuing these red alerts," Hertz said, "but it is certainly not the first time they are taking drastic steps to address their air-quality issues."

China also has become much more transparent about its air quality issues over the last few years, and the red alerts are further evidence of that, Turner said.

"In terms of the transparency of pollution information, this is a very good sign," she said.